Microporous films, particularly polyolefin microporous films are used for microfiltration membranes, battery separators, capacitor separators, materials for fuel cells, and the like, and particularly are suitably used as lithium ion battery separators. In recent years, while lithium ion batteries have been used in application to small-sized electronic apparatuses such as cellphones and laptop personal computers, application of the lithium ion batteries to hybrid electric vehicles and the like has also been attempted.
Here, higher output characteristics to extract more energy in a short time are demanded of the lithium ion battery for hybrid electric vehicles. Moreover, securing of higher safety is demanded because the lithium ion battery for hybrid electric vehicles is generally large-sized, and needs a higher energy capacity. The safety described here is safety such that a battery short accompanied by melting of a resin used for a separator particularly in a high temperature state produced during using the battery is prevented. Here, a temperature when a short occurs within the battery may be referred to as a film rupture temperature of the separator, and a higher film rupture temperature is a measure to realize improvement in safety of the battery.
For the purpose of providing a microporous film that serves as a separator and can be ready for such circumstances, for example, Patent Literature 1 has disclosed a composite microporous film (battery separator) having a laminated layer structure in which a polypropylene microporous film is laminated on a conventional polyethylene microporous film. Moreover, Patent Literature 2 has disclosed a technique to coat a synthetic resin microporous film made of polyethylene with a specific resin porous powder polymer.